Electronics/Battery charging

QuestionHi, I have a question about DC regulator circuits. Is there any way that a circuit can regulate both the voltage at its output and the current at its output irrespective of the load? According to circuit laws, the current consumed by a load is dependent on the resistance of the load right? So can there be any way that we can keep the voltage across the load and the current through the load as a constant?

Thanks.

AnswerIf you were to assume the load on a power supply - or charger, which is a kind of power supply - were a fixed value resistance then ohms law would apply where the voltage is equal to the current times the resistance or E = I * R.

And, yes, you can regulate voltage.
And, yes, you can regulate current.
But, no, you cannot regulate them independently as that would violate ohms law.

But, let's assume you are dealing with battery charger circuits. In actuality the voltage of most batteries does vary somewhat from their discharged state to the fully charged stated. A 12v wet cell, for example, like those used in automobiles and motorcycles, drops to 11.6 or 11.8 volts as they become discharged. At full charge they typically get up to around 13.5v. Therefore, you need a charging device that will keep the voltage going enough to charge up the cells. Further, overcharge of most batteries is dangerous to their electrical health and the charging circuit must 'trickle' off the charge current and let the current drop to zero or a very low value as the battery becomes fully charged.

Charging circuits are usually high impedance drive that allow full charging currents when the battery is low and as the battery becomes charged the current slows down to a trickle.

For power supply circuits there are implementations that can maintain output voltage very precisely up to the maximum output rating of the system. The more expensive ones can keep the voltage constant within 0.001% or even better if necessary. Likewise, there are constant current supplies that will regulate the current to equally accurate levels. Constant voltage supplies are much more common as most circuits like computer cards and microprocessors require a constant operating voltage independent of changes in line voltage, temperature and so on.

There are many types of circuits used to keep the output voltage constant. Here is a discussion about some of them for your review: